The invention relates to “active implantable medical devices” as defined by Directive 90/385/EEC of 20 Jun. 1990 the Council of the European Communities, specifically implants to continuously monitor cardiac rhythm and deliver if necessary to the heart electrical pulses for stimulation, resynchronization and/or defibrillation in case of arrhythmia detected by the device.
Antibradycardia pacing involves the controlled delivery of pulses to the atrium and/or the ventricle. This can be accomplished using single or dual chamber devices. In the case of cardiac resynchronization therapy (CRT), stimulation must also be applied to the two ventricles in conjunction (multisite device). In general, after stimulation of a cavity, it is important to collect the “evoked wave,” that is to say, the wave of depolarization induced by the stimulation of this cavity, to determine whether the stimulation has been effective or not. This test (“capture test”) is used in particular for adjusting the amplitude and/or the width of the stimulation pulses, that is to say, the energy delivered to the stimulation site.
There are many techniques to perform this capture test. Some are described in WO 93/02741A1 or U.S. Pat. No. 5,411,533 A (ELA Medical). One technique is to conduct the stimulation effectiveness threshold test at regular intervals, e.g., every six hours, by implementation of an automatic test algorithm. The amplitude of the stimulation pulse is then adjusted based on the measured threshold and with an extra margin of safety, to take into account uncertainties in the determination of the threshold.
EP1287849 A1 (ELA Medical) discloses a “cycle to cycle” adjustment technique, which includes conducting the capture test and the possible adjustment of the stimulation energy. This technique includes checking at regular intervals (e.g., every six hours) and also continuously checking at each cycle if the stimulation was effective or not.
EP2324885 A1 (Sorin CRM) analyzes endocardial electrogram signals (EGM signals) concurrently collected on two distinct channels taking signals from the same cavity. The two different EGM channels may in particular be that of a unipolar signal (remote or far-field signal collected between the housing and a distal or proximal electrode of the lead), and that of a bipolar signal (close or near-field signal collected between a distal electrode and a proximal electrode of this same lead). Analysis of these signals is a two-dimensional analysis from the “cardiac loop” or “vectogram” (VGM), which is the two-dimensional space representation of one of these two signals relative to the other. This space is typically a “Unipolar channel (y-axis) versus bipolar channel (x-axis)” space, each beat or significant fraction of beat being then represented by its vectogram in the plane thus defined; and therefore ignoring the temporal dimension.
A “vectogram” (VGM), which is obtained from electrogram signals (EGM) from intracardiac leads, is distinct from a “vectocardiogram” (VCG), which is obtained from electrocardiogram signals (ECG) delivered from external electrodes located on the patient's chest.
The analysis of the vectogram for the capture test may be an analysis of the cardiac loop properties. For example, the algorithm calculates and analyzes the descriptor parameters of the vectogram, which may include the angles of the respective tangent vectors considered at various points of the 2D characteristic, or the curvature of this 2D characteristic, or a combination of several parameters, such as a combination of the norm and the angle of the tangent vectors.
Preferably the vectogram analysis is a comparative analysis including a correlation between, first, the characteristics of the vectogram of the cycle to be analyzed, and, second, the same characteristics collected on one or more reference cycles obtained in completely determined conditions: capture, no capture, fusion, etc. For example, the tangent vectors obtained for a cardiac cycle to be analyzed may be compared to the same vectors observed for reference curves, previously obtained, in an identical period, in respectively situations of capture or no capture. Such a characterization algorithm may evaluate a correlation coefficient between the descriptor parameters of the cycle to be analyzed and the reference cycles. The algorithm may discriminate between capture and loss of capture according to the results of the correlation calculation. Such correlation calculations may be combined with other decision criteria, such as the average angle between the respective analyzed vectogram tangent vectors and the reference vectogram. This technique is particularly effective for atypical cycles as in fusion situations, wherein stimulation is initiated concomitantly with spontaneous depolarization during the capture test.
Such a correlation based method is not without drawbacks, however. A first drawback is the level of hardware or software resources necessary for the implementation of the vectogram characterization algorithm. The computational requirements are difficult to reconcile with what is possible to have in a conventional implant, the processor and memory of which are solicited for the implementation of many detection and calculation functions.
A second drawback is the need to have several reference vectograms on which the correlation calculation with the current analysis vectogram is performed. These reference vectograms are obtained either manually, by a test triggered by the practitioner who then validates each reference type (full capture on all stimulated sites, partial capture of certain sites only, total loss of capture, etc.) or automatically. In the case wherein reference vectograms are automatically set, the device regularly performs (e.g., every 4 hours, every week, etc.) high energy stimulation tests or zero volts stimulation tests on the different sites, so as to update the reference vectograms.